Measuring the intraocular pressure (IOP) of an eye is a measurement of the pressure of the fluid inside the eyeball. It is advantageous to monitor IOP as it is an indicator of the health of the eye. Excessively high IOP can be associated with optic nerve damage, such as in the case of glaucoma.
An eyeball may be deemed analogous to an elastic vessel filled with a fluid of a substantially incompressible nature. One can compare such an elastic vessel to a balloon having extensible walls wherein an increase in volume in the fluid produces a change in the internal pressure balanced by an expansion of the vessel wall. Fluids inside the eye circulate in a substantially continuous fashion and an increase in the influx of fluids normally accompanies a similar increase in the outflow of fluid. In cases where the outflow does not keep up with inflow, an increase in internal pressure and an expansion of the eye will occur. In situations where the rigidity of the eye's wall is increased, two effects are observed: increases in the internal pressure are greater per increase in fluid inflow; and a smaller overall expansion of the volume of the eye.
The change in the expansion of the eye depends on the extensibility of the walls of the eyeball. The more extensible the wall, the greater the ability for eye's volume to increase in response to a fluid volume change. The less extensive is the wall, the less capable is the eyeball to cope with fluid volume change and the more the fluid pressure will increase.
Typically, in biomedicine, pressure such as IOP is not measured directly because of the invasive nature of placing a pressure sensor in the fluid of the eyeball. Therefore, determination of pressure is typically attempted using alternate less invasive methods. Consequently, while measuring intraocular pressure directly, continuously, and non-invasively is desired, it is difficult to achieve.
Moderately invasive measurements are known and have already been conducted. Devices known as “Contacting Tonometers” have been used extensively by the medical community for many years. However, their popularity and attractiveness is offset by the need to have direct mechanical contact with the eye, thus requiring an anesthetic. Further, the requirement for contact and the resulting deformation of the eye can introduce errors in the determination of IOP due to formation of tears, changes in eye volume due to compression, and as a result the variance of the physical properties of the cornea. Such prior art devices are described in U.S. Pat. Nos. 2,519,681; 3,049,001; 3,070,087; 3,192,765.
Various other attempts have been made to measure IOP discreetly or continuously by means of more indirect methods. Indirect methods have the advantage of being non-invasive, or at least less invasive than indentation and applanation tonometry. As set forth in U.S. Pat. No. 3,181,351, one such method introduces a sharp pulse of air onto the eye, while measuring the resulting deformation of the cornea. Such indirect methodology usually suffers from two limitations: lack of accuracy and a lack of absolute value in resulting measurements.
Both the invasive and indirect methods assume that all eyes have substantially equivalent physical properties. This is not the usual case. What is required is a means to determine IOP which is cognizant of the variability of characteristics from eye to eye. Some have attempted to relate IOP to an eye's mechanical response to gross stimulus and nominal eye characteristics and others have associated IOP with measures of eye geometry.
In contradistinction to the prior art, Applicants have sought to determine both characteristics specific to the structure of the eye and characteristics which vary as a result of changes in IOP.